Huntington disease (HD) is an autosomal dominant neurodegenerative disorder usually characterized by onset in adult life with psychiatric disturbances, progressive involuntary movements and dementia. The genetic defect underlying HD has been mapped to chromosomal band 4.sub.p 16.3. Allelic association studies with genetic markers and assessment of specific recombination events in affected families to further refine its location have provided conflicting results. These results place the gene either at a proximal location within a 2.2 Mb fragment or in a more distal non-overlapping DNA segment close to the telomere of 4.sub.p. Despite some differences in published reports of allelic association, all studies have consistently demonstrated nonrandom association between the gene causing HD and a DNA marker at D4S95 which is located in the proximal 2.2 Mb candidate region approximately 1.2 Mb telomeric to D4S10, the locus first found to be linked to HD.
The biochemical defect underlying HD is not known and therefore current strategies to identify the HD gene have necessitated assessment of genes located in both of the candidate regions. Cloning genes from these large regions has in the past been limited by the difficulties in rapidly identifying coding regions over large stretches of genomic DNA. More recently however, a variety of techniques including exon amplification and direct cDNA selection strategies have been developed which greatly facilitate the search for coding sequences in genomic DNA. We have isolated and cloned the cDNA for human brain or .alpha.-adducin gene, a portion of which was initially detected by using a modification of a direct cDNA selection strategy. This gene has been localized to within 20 kb of D4S95.
Adducin is a membrane associated cytoskeleton protein comprised of .alpha. and .beta. subunits. Adducin in solution is a heterodimer, and is present in membranes of a wide variety of cells including erythrocytes and neurons. The erythrocyte cytoskeleton has been shown to have a lattice-like organization with actin complexes crosslinked by spectrin molecules extending throughout the entire membrane skeleton. Adducin is thought to play a role in the assembly and maintenance of the actin-spectrin junctional complex and promotes binding of spectrin to actin. Adducin was initially identified on the basis of its calmodulin binding activity which inhibits its ability to promote the binding of spectrin with actin. Furthermore, .alpha.-adducin is phosphorylated by cAMP-dependent protein kinase and is also a major substrate for protein kinase C. Brain tissue contains an isoform of adducin in lower concentration than in erythrocyte membranes but with similar properties including the association with spectrin-actin complexes, calmodulin binding and phosphorylation by protein kinase C.
The functional consequences of defects in the adducin gene are unknown. However mice deficient in ankyrin, a related cytoskeletal protein with spectrin binding properties have, in addition to hemolytic anemia, significant neurological dysfunction associated with Purkinje cell degeneration in the cerebellum and the development of a late onset neurological syndrome characterized by persistent tremor and gait disturbance. Ankyrin and adducin appear to have different functions in the membrane skeleton but both play a role in the interaction with spectrin and the maintenance of normal membrane integrity. Moreover, previous studies of red cells, fibroblasts, lymphocytes and neurons in affected HD patients have pointed to a possible generalized disturbance in membrane structure and function in this disorder.